1) Field of the Invention
The present invention relates to ceramic packages, made of a silicon nitride sintered body having a large heat conductivity, for containing a semiconductor chip. The ceramic packages according to the invention have a high heat dissipation, and can be applied to various types of ceramic packages, such as pin-grid array type ceramic packages, chip-carrier type ceramic packages, cer-dip type ceramic packages, cer-quad type ceramic packages, and flat type ceramic packages. The present invention also relates to ceramic for a wiring board having a high heat dissipation.
2) Description of the Related Art
Recently, electric circuits have been highly integrated on semiconductor chips in a very large scale and operations of such electronic circuits are executed at very high speed. The amount of heat generated in a semiconductor chip when the electronic circuits formed on the semiconductor chip are operated seems to increase more and more. Particularly, in semiconductor chips having bipolar type circuits, such a tendency is found. Therefore, ceramic packages and ceramic wiring boards made of a material which can radiate the heat generated in the semiconductor chip well, i.e. a material having a large heat conductivity, are required.
Hitherto, ceramic packages made of alumina are widely used for containing such semiconductor chips that generate a large amount of heat, because alumina ceramic is more reliable and has better heat dissipation than resin.
FIG. 12 is a schematic view showing a known pin grid array type ceramic package made of alumina ceramic. On the pin grid array type package 81, a semiconductor chip 82 is mounted; a metal conductor wiring is arranged inside of the package 81; and a plurality of metal pins 83 are formed to be aligned on both sides of the package 81 as terminals for contacting the semiconductor chip 82 to the outer circuits. Further, a chip carrier type package, in which metal lead wires are used instead of the metal pins 83, made of alumina ceramic is also well known.
However, the heat dissipation of the alumina ceramic package is not sufficient to radiate the great amount of heat generated in the semiconductor chip contained therein. Accordingly, a package for containing a semiconductor chip having a large heat dissipation is strongly required. Further, a thermal expansion coefficient of the package made of alumina ceramic does not match with that of silicon, which is used as a material of the semiconductor chip. Thus, there are problems that a thermal stress is sometimes caused in the chip and the chip is thereby destroyed.
In order to solve such problems, aluminum nitride ceramic, which has better heat conductivity and whose thermal expansion coefficient matches with that of silicon, has started to be used as a material of the ceramic packages. However, the aluminum nitride ceramic may be damaged from alkali and water. Furthermore, mullite ceramic, whose thermal expansion coefficient matches with that of silicon, has also begun to be used as a material of the ceramic package. However, the heat conductivity of mullite ceramic is not sufficient, so that ceramic package made of mullite cannot radiate the heat generated in the semiconductor chip contained therein efficiently.
In order to solve these problems, ceramic packages comprising a heat radiating plate and a wiring board have begun to be used. The construction of such packages are illustrated in FIGS. 1 and 2. In these packages, a ceramic material having a large heat dissipation is used for the heat radiating plate 1, on which the semiconductor chip is mounted, in order to radiate the heat generated in the semiconductor chip; and alumina ceramic or mullite ceramic is used for the wiring board 4.
Alumina ceramic or mullite ceramic is industrially used in large quantities and the cost is therefore cheap. Further, there are such merits that alumina ceramic products or mullite ceramic products can be produced at a low sintering temperature and a process conventionally used for producing alumina or mullite ceramic packages can be used as it is. When signals are transferred through the wiring circuit at a high speed or when a resistor of a power supply circuit is required to be made small, a conductor having a small conducting register, i.e. metal of Ag series, Cu series or Au series is used for the wiring. Therefore, it is required to use ceramics which can be sintered at a low temperature of less than 1100.degree. C., i.e. a temperature less than the melting points of these metals, for the wiring board.
There is further suggested a package comprising a wiring board, in which an insulating material is made of polyimide resin and a conductor pattern is formed by a photo lithographing a thin film formed thereon by sputtering, depositing or coating; and a heat radiating plate made of aluminum nitride or silicon carbide into which BeO is added.
In order to contact the heat radiating plate and the board including a wire distribution inside thereof, glass, Au, Cu, or an activated metal, which is made by adding Ti or Zr into an Au alloy or into a Cu alloy, is used. Or, conductors formed on the respective boards for the purpose of electric connection are sometimes connected to each other by means of a metal solder or by sintering a metal paste arranged between the conductors. However, when alumina ceramic is used as the insulating material of the wiring board, since the thermal expansion coefficient of the alumina ceramic does not match with that of the silicon carbide, into which BeO is added, and with that of aluminum nitride used as the insulating material of the heat radiating plate, a crack would be caused in the contacting portions of these plates due to thermal stress. In such a case, it is necessary to form a ceramic frame having an intermediate thermal expansion coefficient between both of the plates.
Aluminum nitride, which is used as the insulating material of the heat radiating plate, does not prevent damage from environments such as water and alkali, so the reliability of the package as a whole is detracted. Further, silicon nitride also has a problem that its withstand voltage is low, because there exists an insulating characteristic only at grain boundary portions in the silicon nitride body.
Further, cer-dip type, cer-quad type or flat type package for containing a semiconductor chip is also well known. The structure of the package of such type is shown in FIG. 4. The package comprises a ceramic base 13 and a ceramic cap 14, which is arranged on a ceramic base 13 and contacted to each other so as to seal up a semiconductor chip 11, to cover it a conductive layer 12 made of Au, by means of a bonding agent such as glass or resin. There is provided a metal lead frame 17, which is for electrically connecting the semiconductor chip 11 to the outer circuits.
However, when using alumina as a material of such type package, since the fracture toughness of alumina is low, edges of the package are apt to be broken. Further, there are problems such that the heat dissipation of alumina is not sufficient to radiate the heat generated in the semiconductor chip contained in the package and that the thermal expansion coefficient of alumina is not coincident with that of silicon, as stated above. Alumina nitride or silicon carbide, into which a little amount of BeO is added, has begun to be used as a material of the package of such type.
Aluminum nitride ceramic has a thermal expansion coefficient being coincident with that of silicon, but it does not prevent damage from water and alkali, as mentioned above. Silicon carbide ceramic into which a little amount of BeO is added, has an insulating characteristic only at grain boundaries, the withstand voltage by the silicon carbide ceramic is low. Furthermore, the density of the silicon carbide ceramic is not made high by general sintering but by hot press sintering. Thus, the processing cost for manufacturing packages made of the silicon carbide ceramic becomes very high.
Hitherto, as the material of the ceramic wiring board alumina ceramic is widely used, because the base plate made of alumina ceramic has a high reliability and a high heat dissipation in comparison with the board made of resin. However, the alumina ceramic board is worse than the board made of resin regarding a practical strength. Additionally, the heat dissipation is not sufficient to radiate the heat generated in the semiconductor chip, in which a great number of circuits are highly integrated.
In order to improve the heat dissipation of the board, aluminum nitride ceramic, which has a higher heat conductivity, and silicon carbide, into which is added a little amount of BeO, has begun to be used as a material for the board.
However, as mentioned in the above, aluminum nitride ceramic does not prevent damage from the environment, such as water and alkali. Additionally, since the heat expansion coefficient of aluminum nitride ceramic is greatly different from that of metal, portions of the ceramic base plate which contact metal pins or metal lead wires therewith are apt to be broken due to stress caused in the board due to the difference between the heat expansion coefficients.
Further, when using silicon carbide to which has been added a little amount of BeO, there is a problem in electrical characteristics, such that its dielectric constant is large, i.e. about 40. Additionally, silicon carbide does not prevent damage from voltage and the processing cost for manufacturing a board made of silicon carbide becomes high because hot press sintering is necessary to make the density thereof high.